This invention relates to multiplex transmission systems and, more particularly, to the switching of time-division multiplexed signals through space-division telephone central offices.
In the past, it has been proposed to digitally switch the telephone calls carried by time-division multiplex lines, such as the well-known T1 carrier system manufactured by the Western Electric Company, through conventional space-division switching offices. For example, A. E. Joel, Jr. U.S. Pat. No. 3,652,803 issued Mar. 28, 1972 teaches that time-division multiplex signals may be demultiplexed and digitally transmitted cross-office over as many physical, i.e., space-division, paths as there are time-division multiplex channels in a frame; provided that each such space-division path has a bit repetition rate low enough to avoid crosstalk. Of course, the bit repetition rate must be high enough so that all of the information is transmitted cross-office before the end of the frame interval.
In a conventional space-division telephone switching office a cross-office transmission path check is performed incident to the setting up of the connection and prior to the placing of the path in the talking state. This test verifies the integrity of the transmission path extending from the incoming trunk over the network crosspoints to the outgoing trunk. As described in either A. J. Busch U.S. Pat. No. 2,585,904 issued Nov. 12, 1952 (relating to the well-known No. 5 crossbar system) or in the Sept. 1964 issue of the Bell System Technical Journal (relating to the No. 1 Electronic Switching System), such a continuity check is accomplished under the direction of common control. In a crossbar system the common control is known as a marker; in an electronic switching system the common control is known as a processor. In either case, the off-hook state of the trunks is verified and when successfully completed, common control allows the call to be cut-through to the talking state. In both cases the continuity check is a D.C. check.
In the aforementioned Joel patent system each of the cross-office channels employs a digital trunk circuit at each of its ends. The digital trunk circuit includes shift registers and flip-flops for storing and responding to the digital content of the transmitted information. The conventional D.C. transmission path check performed by the central office for analog trunks provides no indication of how these digital components in the cross-office channel are functioning.
In a conventional T1 carrier, analog-switched transmission system, digital transmission integrity is verified by a technique known as loop-around testing according to which the output of a digital-to-analog decoder and the input of an analog-to-digital encoder at the same end of the transmission channel are temporarily bridged together. A digital test code applied at the decoder input and looped around the temporary bridge is compared to the code received at the encoder output. This prior art technique requires manual intervention in system operation and does not pinpoint the trouble which may lie at any one of many cross-office trunk channels employing the aforementioned encoder-decoder circuit in common.
It would be advantageous to provide, on an in-service basis, an arrangement for automatically verifying the digital transmission properties of individual cross-office channels or trunks across a space-division switching office on an economical and determinative basis.
Prior art transmission path checks on a per-digital channel basis are known in time-division switching networks wherein a channel shifting circuit time-domain matches the two time slots corresponding respectively to the calling and called line circuits. Before the called line circuit is placed in the talk state, a digital test code is applied to the channel shifting circuit and is routed through the network crosspoints associated with the called line circuit, through the called line circuit itself (which has temporarily bridged its transmit and receive portions) and back again through the network crosspoints to another shifting circuit. The received and transmitted test codes are then compared at the processor. This system, however, only checks the called circuit and does not test for network integrity on an end-to-end basis. In addition, the loop-around techniques for continuity path testing in time-division switching systems make use of the fact that the receive and transmit transmission directions are synchronously clocked, thereby allowing the bridging and looping around of a digital test code. Such bridge-and-loop-around techniques are, however, not applicable to the space-division switching system described in the Joel patent which does not require that the common channel multiplexer and demultiplexer be in respective frame synchronization. The aforementioned testing arrangement is thus not readily applicable to a space-division switching network without the inclusion of transmit-receive bridging circuits, comparator equipment and memory buffering at each end of a digital trunk and without the consumption of valuable real time during call processing cycles of the common controller.
It would therefore be advantageous to provide a transmission path continuity check in time-division multiplex systems over a space-division switching network which tests individual trunks on an end-to-end basis without requiring additions of loop-around circuitry, of buffering memories or of processor control real time.